In optical communication systems, messages are transmitted by carrier waves of optical frequencies that are generated by sources such as lasers or light-emitting diodes. There is much curent interest in such optical communication systems because they offer several advantages over certain other conventional communication systems, such as a greatly increased number of channels of communication and the ability to use other materials besides expensive copper cables for transmitting messages. One such means for conducting or guiding waves of optical frequencies from one point to another is called an "optical waveguide." The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus producing a light guiding effect.
In a multimode fiber optic system, in which multiple electric field profiles shape the light fields, couplers are necessary, for example, to achieve parallel distribution of data by means of optical fibers from a single source, such as a computer, to multiple end points, such as terminals, or from multiple end points to a single source. Current approaches to optical coupler formation include processes which use microoptic components such as microlenses and splitters or processes which manipulate the optical fibers, such as by fusing, tapering, lapping, or gluing. The former approach has the disadvantage that lenses must be cut and polished to specification, which are costly processes. In the latter approach of manipulating the optical fibers, there is the disadvantage of difficulty in contolling the extent of contact of the fibers and the exact geometry and reproducibility of the final structure.
Another approach to the problem of coupling optical signals between certain locations or points is to use a star coupler device consisting of a cylindrical glass rod having within it and accessible at each end, two bundles of glass fibers which terminate within the glass rod at a point such that the two separate bundles do not contact each other. An optical input to any one fiber of one bundle of fibers is propagated through the glass rod and transmitted as an output by all fibers in the second bundle of fibers. However, the major disadvantage of this approach is that there are packing fraction losses which are caused by propagated light striking the clad of the fiber rather than the core of the fiber. Since light is guided only by the core of the fiber, any light which falls on the surrounding clad of the fiber is lost and ineffective.
Another star coupler device known in the art uses a thin (approximately 65 micrometers) glass slide with a linear array of optical fibers abutted to each end of the slide. This star coupler device functions in a manner similar to that described above with respect to the rod device and has the same significant disadvantage of packing fraction losses. For example, if 8 fibers having a core with a diameter which is one-half the total fiber diameter are used in a linear array, by placing them side by side in a ribbon shape, the theoretical packing fraction loss is 42 percent, that is, the expected optical output is reduced by 42%.
In a related area of merely coupling two optical signals together, a Y-shaped optical coupling device has been used and is fabricated by conventional planar processing. Such a device is disclosed in copending application Ser. No. 25,709, filed Apr. 2, 1979, assigned to the present assignee. Other horn structures are known in the art and have been used in single mode optical waveguides to expand the electric field profie of an optical signal. However, these single mode optical waveguides do not perform the data distribution function discussed above with respect to star couplers.